This invention relates to a photo-electric conversion tube such as a photomultiplier tube or photoelectric tube.
In the prior art the vacuum container of a photoelectric conversion tube has an incident light window made of a transparent plate such as a quartz glass plate or borosilicate glass plate which can withstand the atmospheric pressure. Additionally, a translucent photocathode surface of antimony and alkali metal is formed on the inner wall (vacuum side) of the incident light window. In operation a light beam reaching the photocathode surface through the incident light window is absorbed by the photocathode surface, where it is converted into photoelectrons. The photoelectrons are emitted towards the vacuum side.
FIG. 1 shows an optical characteristic of a multialkali photocathode surface which is one example of the photocathode surface. In other words, FIG. 1 is a graphical representation indicating wavelength (.lambda.) with absorption coefficient (k) for a multi-alkali photocathode surface. As is apparent from the graphical representation, the multi-alkali photocathode surface has a small absorption coefficient (k) in the range of long wavelengths, especially in an infrared range; that is, it cannot sufficiently absorb light in the range.
FIG. 2 is also a graphical representation indicating optical characteristics of photocathode surfaces; i.e., thicknesses (d) with absorption coefficients (k) with wavelengths (.lambda.) as parameters. As is apparent from the graphical representation, when the thickness (d) of a photocathode surface is increased to increase the photo-electric conversion efficiency, the absorption coefficient is also increased. However, in this case, the distance between the place where photoelectrons are generated and the vacuum side increases, as a result of which some of the photoelectrons undergo recombination while moving towards the vacuum side, and the percentage of photoelectrons emitted into the vacuum; that is, the photo-electric conversion efficiency is decreased. Thus, the increase in thickness of the photocathode surface is limited.
For instance, an ordinary multi-alkali photocathode surface is 30 nm in thickness, and its absorption percentage of a light beam having a wavelength of 800 nm is 12%. And the distance for which photoelectrons generated move until they are recombined is about 15 nm, and therefore the quantum efficiency (a ratio of the number of photo-electrons to the number of incident photons) is 1.5% When a photo-electric conversion tube having the above-described characteristic values is employed as a scientific instrument, its S/N ratio is not so high.
As was described above, in the conventional photo-electric conversion tube, the photocathode surface cannot absorb an incident light beam because the thickness is limited, or the rate of emitting photoelectrons in the vacuum is low for the same reason.